Subsea cooling system

ABSTRACT

A subsea hydrocarbon cooling and heat transfer system is provided. The system includes one or more subsea hydrocarbon process fluid heat exchangers arranged in heat exchange communication between a hydrocarbon process fluid and a cooling medium fluid, a cooling medium fluid distribution pipe system connected to the hydrocarbon process fluid heat exchangers, and a subsea cooling unit for cooling the cooling medium fluid. After passing through one of said heat exchangers, a heated cooling medium can act as a heat transfer fluid to re-heat another hydrocarbon process fluid.

FIELD

The present invention relates to a subsea hydrocarbon cooling system, asubsea cooling medium and heat transfer system, and related subseaprocesses for facilitating the cooling, separating and onward processingof hydrocarbons.

BACKGROUND

Cooling systems are essential for several stages of hydrocarbonprocessing. For example, it is typical to cool hydrocarbon gas fromwellhead temperatures, which commonly range from about 80° C. to about150° C., down to about 30° C. to about 60° C., prior to various offshoretopsides processing steps, including dehydrating the hydrocarbon gasand/or separating condensates therefrom, and as part of compression.

Subsea applications for hydrocarbon cooling generally occur where thehydrocarbons are sourced from subsea wells, as distinct from ‘dry tree’platform wells, when the wells are brought up to either fixed orfloating platform facilities. Cooling has been required to meet maximumpipeline design temperature, (to prevent pipeline upheaval buckling, andto prevent excessive corrosion) particularly in high pressure, hightemperature (HP/HT) wells. Another subsea cooling application occurs aspart of subsea compression projects.

In the future, as there is expected to be greater demand to conduct moreprocessing subsea, as compared to on fixed or floating platformfacilities, particularly in deepwater and remote locations, there willbe a greater demand for improved subsea hydrocarbon cooling systems.

Various devices have been proposed for cooling hydrocarbons subsea.

The cooling devices generally send the hydrocarbons down either a singleexposed pipe or a network of pipes from which cooling results fromenergy loss through the pipe wall to the surrounding sea. There areseveral significant disadvantages from current proposals:

-   -   There is no real mechanism to accurately control the hydrocarbon        temperature, which will naturally vary with flowrate. Where        cooling is too great and the hydrocarbon fluid flow is low there        is a risk of hydrate formation.    -   Given the lack of ability to control temperature, separation        processes to remove water or condensate at a given temperature        and pressure have not been available subsea. To date these        processes have been confined to topsides facilities only.    -   The pipe network is invariably made from a high alloy stainless        steel or equivalent which can be prohibitively expensive.    -   Apart from hydrates, the pipe cooling network can be        pre-disposed to build up of wax and sometimes sand in periods of        reduced flowrates and operating temperatures. These potential        blockages are difficult to remove from the pipe network        configuration.

Other proposed subsea cooling devices look to enhance the cooling dutyobtained and provide temperature control. This is proposed by pumpingseawater, in various configurations and by various devices, across thepipe-network containing the hydrocarbon fluids.

The pumped seawater passes across the pipes at an increased velocity.This in theory achieves a higher forced-convection external pipe heattransfer coefficient and may achieve temperature control by varying theseawater velocity. While the principle of this heat transfer method iscorrect, in practice this method will reduce in effectiveness if anymarine fouling or scale coating develops over time. Loss of temperaturecontrol may result.

The present invention seeks to overcome at least some of theaforementioned disadvantages.

SUMMARY

In its broadest aspect, the invention provides a subsea hydrocarboncooling system, a subsea cooling unit, and a method of cooling a subseahydrocarbon process fluid, by way of a cooling medium fluid.

According to a first aspect of the invention there is provided a subseahydrocarbon cooling system, said system comprising:

-   -   one or more hydrocarbon process fluid heat exchangers arranged        in heat exchange communication between a hydrocarbon process        fluid and a cooling medium fluid, wherein the one or more        hydrocarbon process fluid heat exchangers are located subsea;    -   a cooling medium fluid distribution pipe system connected to the        hydrocarbon process fluid heat exchangers; and,    -   a subsea cooling unit for cooling the cooling medium fluid, the        subsea cooling unit comprising:        -   an inlet and an outlet arranged in fluid communication with            the cooling medium fluid distribution pipe system;        -   one or more subsea cooling modules, each cooling module            comprising a plurality of cooling pipes configured in heat            exchange relationship with surrounding seawater;        -   a first conduit for providing fluid communication between            the inlet and the one or more subsea cooling modules; and        -   a second conduit for providing fluid communication between            the one or more subsea cooling modules and the outlet.

The cooling system may be provided with a pump to circulate the coolingmedium fluid through the hydrocarbon process fluid heat exchangers andthe subsea cooling units of the cooling medium system.

According to another aspect of the invention there is provided a subseacooling unit for cooling a cooling medium fluid circulating through asubsea heat exchange circuit, the subsea heat exchange circuit having acooling medium fluid distribution pipe system, the subsea cooling unitcomprising:

-   -   an inlet and an outlet arranged in fluid communication with the        cooling medium fluid distribution pipe system;    -   one or more subsea cooling modules, each cooling module        comprising a plurality of cooling pipes configured in heat        exchange relationship with surrounding seawater;    -   a first conduit arranged to provide fluid communication between        the inlet and the one or more subsea cooling modules; and    -   a second conduit arranged to provide fluid communication between        the one or more subsea cooling modules and the outlet.

In a further aspect of the invention there is provided a method ofcooling a subsea hydrocarbon process fluid, said method comprising:

-   -   passing the hydrocarbon process fluid through a subsea        hydrocarbon process fluid heat exchanger and passing a cooling        medium fluid through a cooling medium fluid distribution pipe        system, wherein said subsea heat exchanger and said pipe system        are configured to provide heat exchange communication between        the hydrocarbon process fluid and the cooling medium fluid,        thereby producing a cooled hydrocarbon fluid and a heated        cooling medium fluid; and,    -   cooling the heated cooling medium fluid by passing the heated        cooling medium fluid through a subsea cooling unit, wherein the        subsea cooling unit comprises:        -   an inlet and an outlet arranged in fluid communication with            the cooling medium fluid distribution pipe system;        -   one or more subsea cooling modules, each cooling module            comprising a plurality of cooling pipes configured in heat            exchange relationship with surrounding seawater;        -   a first conduit arranged to provide fluid communication            between the inlet and the one or more subsea cooling            modules; and        -   a second conduit arranged to provide fluid communication            between the one or more subsea cooling modules and the            outlet.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of one embodiment of a subseahydrocarbon cooling system of the present invention;

FIG. 2 is a schematic representation of another embodiment of the subseahydrocarbon cooling system of the present invention; and,

FIG. 3 is a schematic representation of an embodiment of a subseacooling module associated with the subsea cooling unit of the presentinvention.

DETAILED DESCRIPTION

Referring to the Figures, where like numerals refer to like featuresthroughout, there is shown a subsea hydrocarbon cooling system 10.

The subsea cooling system 10 is generally sea-based, but could equallyapply to any body of water including inland or lake-based water bodies.It will be appreciated that a reference to a sea floor, sea bed, orseawater may equally apply to a lake floor, lake bed, or lakewaterand/or freshwater and/or saltwater and/or brine, respectively, dependingon the location of the offshore subsea cooling system and the characterof the body of water in which it is located.

The subsea cooling system 10 may be associated with a hydrocarbonprocessing area 12 located on, or proximal to, a seabed 14. Thehydrocarbon processing area 12 is configured to process hydrocarbonfluids in accordance with subsea processing requirements, such as subseacondensate and/or water separation and subsea compression processes andwill have a need for hydrocarbon fluid cooling. Accordingly, it will beappreciated that one or more of the subsea cooling systems 10 of thepresent invention may be conveniently integrated into the hydrocarbonprocessing area 12.

The term ‘hydrocarbon fluid’ refers to a gas, liquid, or dual phaseliquid-gas stream containing one or more hydrocarbons. The hydrocarbonmay be extracted directly from a well head in the form of a liquid, forexample in the form of crude oil, or as a gas, for example in the formof natural gas, or as a mixture of natural gas and crude oil.

The system 10 includes one or more heat exchangers 16, 16′ arranged inheat exchange communication with a hydrocarbon fluid. The heatexchangers 16, 16′ may be of various types, such as shell & tube, orvarious others well understood by those skilled in the art, and may bearranged in various configurations (series/parallel), locations, andnumber as part of the overall hydrocarbon processing area 12. It will beappreciated that the heat exchangers 16, 16′ may comprise conventionalshell & tube heat exchangers which have been modified for subsea use.

The system 10 also includes a heat exchange circuit 18 disposed inparallel heat exchange relationship with the heat exchangers 16, 16′. Acooling medium fluid circulates through the heat exchange circuit 18.FIGS. 1 and 2 show alternative schematic representations of therelationship between heat exchange circuit 18 and heat exchangers 16,16′. In general, the hydrocarbon fluid will be communicated to the heatexchanger 16, 16′ from a processing unit within the hydrocarbonprocessing area 12 via conduit.

The temperature of the cooling medium fluid is less than the temperatureof the hydrocarbon fluid. When the hydrocarbon fluid is passed throughthe heat exchanger 16, thermal energy from the hydrocarbon fluid istransferred to the cooling medium fluid which is circulated through theheat exchange circuit 18. Consequently, the hydrocarbon fluid is cooled.

The cooling medium fluid may be any suitable fluid which is capable offlowing through the heat exchange circuit 18 and transferring heat froma fluid, such as a hydrocarbon fluid, via the heat exchanger 16, 16′.Preferably, the cooling medium fluid has a high thermal capacity, lowviscosity, is low cost, non-toxic, and chemically inert, neither causingnor promoting corrosion of the heat exchange circuit 18.

In general, the cooling medium fluid of the present invention may be aliquid, although in some alternative embodiments of the invention thecooling medium may be a gas.

Suitable examples of cooling medium fluids include, but are not limitedto, aqueous media containing additives to inhibit corrosion within theheat exchange circuit 18, depress the melting point and/or raise theboiling point. In a preferred embodiment the cooling medium fluidcomprises water mixed with a suitable organic chemical, such as ethyleneglycol. It is possible that the fluid is the same fluid as used inwater-based subsea hydraulic control systems, as this should also meetthe requirements for heat transfer.

Both the heat exchangers 16, 16′ and the heat exchange circuit 18 aredisposed subsea, preferably supported by mud mats 20 or other suitablestructural foundations on the seabed 14. The mud mats 20 may be simplestructural steel pallets.

It will be appreciated that the heat exchanger 16, 16′ and the heatexchange circuit 18 will be fabricated from materials which are suitablefor use in a subsea environment and which are corrosion resistant. Forexample, the shell of the heat exchanger 16 may be fabricated fromcarbon steel, with appropriate cathodic protection systems and coatingsto resist external corrosion. The heat exchanger 16 tubing may be formedfrom a suitable corrosion resistant alloy (CRA), stainless steel orequivalent material suitable to resist corrosion from the hydrocarbonfluid.

Those skilled in the art will know the various factors that determinewhich fluid, be it the process hydrocarbon fluid or the cooling mediumfluid, pass through the shell or the shell & tube exchanger and whichpass through the tubing. It is likely that the process hydrocarbon fluidwill pass through the tubing, such that the shell can be formed fromcarbon steel. This arrangement is also preferred as it is likely tominimise the fouling potential of the hydrocarbon process fluid.However, it will be understood that the preferred arrangement may varydepending on other factors, such as the hydrocarbon composition andprocessing requirements.

The system 10 of the present invention also includes a subsea coolingunit 22 for cooling the cooling medium fluid. In use, the cooling mediumfluid must be continuously cooled for reuse as a heat transfer fluid asit circulates through the heat exchange circuit 18.

The subsea cooling unit 22 includes an inlet 24 and an outlet 26 whichare connectable to the heat exchange circuit 18, and one or more subseacooling modules 28. The one or more subsea cooling modules 28 arearranged in fluid communication with the inlet 24 and the outlet 26 viaa first conduit 30 and a second conduit 32, respectively. The first andsecond conduits 30, 32 may be lengths of hard pipe or flexible pipe.

The flow of cooling medium fluid (and hence the degree of coolingachieved) through heat exchange circuit 18 may be controlled via a flowcontrol valve (not shown) in the subsea cooling unit 22. Alternatively,flow may be controlled with a pump 34, such as a single phase variablespeed pump, which is also located subsea. Pump service is not expectedto be onerous due to the controlled, clean and single phase nature ofthe cooling medium fluid.

The cooling module 28 comprises a plurality of cooling pipes 36configured in heat exchange relationship with surrounding seawater. Theplurality of cooling pipes 36 may be configured in a simple network offully-welded small bore pipe lengths, typically of about 1½-2 inchdiameter. The inventor estimates that approximately 250 m of 2 inch pipewould provide a typical cooling duty of around 0.25 MW, and cool 1500bpd (barrels per day) or 240 m³/h of cooling medium by approximately 25°C. A manifolded unit of 10 lengths of such pipe in parallel would have acooling duty of about 2.5 MW. Several manifolded units could be combinedin the cooling module 28.

It will be appreciated that the diameter, individual lengths of pipe,number of manifolded units and cooling modules may vary and be optimisedaccording to the desired design and performance requirements as well asambient conditions.

Alternatively, the plurality of pipes 36 may be configured in a coiledarrangement or any other number and type of shapes and arrangements inheat exchange relationship with the surrounding seawater.

The plurality of pipes 36 may be coiled around at least part of asubstructure of a subsea facility, such as a valve manifold, part of asubsea compression module, or even part of the substructure of anoffshore platform.

Further, it will be appreciated that further cooling to lowertemperatures may be possible with longer lengths.

Efficiency, in terms of heat loss per unit length of pipe decreases witha lower temperature differential between the cooling medium and theambient seawater. It is envisaged that in some embodiments, a portion ofthe cooling medium could be cooled or chilled to a lower temperature,with series units, in addition to, or alternatively to, the parallelarrangement described above. Valve isolating sections of the coolingmodule het exchange may be used to establish a minimum operatingtemperature of the cooling medium fluid. This may be advantageous toensure no ‘cold spots’ exist in the hydrocarbon process fluid heatexchanger. Other options may be preferred, such as operating thehydrocarbon process fluid heat exchanger in co-current instead of themore usual counter-current configuration. The detailed design option mayvary depending on the particular installation and processingrequirements for the hydrocarbon fluid.

The one or more subsea cooling modules 28 are preferably located abovethe sea bed 14. Sea currents are reduced very close to the sea bed 14.Therefore, locating the cooling modules 28 some metres above the sea bed14 may have some advantage of exposing the cooling modules 28 tostronger sea currents. Stronger currents may generate some improvementin the efficiency of heat transfer from the cooling modules 28, althoughinherent thermal convection may largely contribute to adequate thermalenergy transfer when sea currents are weak or absent.

The cooling module pipes 36 may be prone to some marine growth and scaleformation. This will depend on location, ambient conditions,temperature, and water depth. Any marine growth and scale will reducethe efficiency of heat transfer in the subsea cooling modules 28 overtime. This may also reduce the benefit of increased seawater velocityfrom currents or other sources. The reduced heat transfer can becountered by installing a greater number (or length) of pipes 36 thanrequired. Additionally, the marine growth may be removed intermittentlywith suitable removal methods, such as by blasting with a high pressurewater jet, to preserve the heat transfer capability of the coolingmodule pipes 36.

The subsea cooling modules 28 may be supported by ‘mud mats’ 20′ orother structural foundations, residing on the sea bed 14. In thisparticular embodiment, the spacing between the cooling modules 28 andthe sea bed 14 may be determined by the height of the mud mats 20′ whichsupport the cooling modules 28. The mud mats 20′ may be simplestructural steel pallets. In some embodiments, the mud mat 20′ couldincorporate lengths of large diameter pipe through which the coolingmedium fluid could also be circulated and/or distributed to the coolingmodules 28.

In some embodiments, as shown in FIG. 3, the subsea cooling modules 28may be provided with an open-ended shroud 50 to direct flow of seawaterover the subsea cooling modules 28 and/or increase thermal convectionfrom the subsea cooling modules 28. The shroud 50 may be provided withbaffles 48, vanes or other similar means for increasing and/or directingflow of seawater over the subsea cooling modules 28. It will beappreciated by persons skilled in the art that the shroud 50 and baffles48 illustrated in FIG. 3 are representational. The number, type andshape of the shrouds 50 and baffles 48 used in respect of the subseacooling modules 28 may vary depending on desired operationalrequirements and they may be optimised by well understood designprinciples.

In some embodiments, the subsea cooling modules 28 may be locatedremotely (i.e. several kilometres) from the subsea hydrocarbonprocessing area 12 in deeper and colder seawater. In this embodiment thefirst and second conduits 30, 32 may be in communication with thecooling modules 28 via respective seabed pipes. Colder seawater maysignificantly improve cooling efficiency as will the length of seabedpipe. The inventor notes that such additional cooling benefits wouldhave to outweigh the costs associated with fabricating and installingoffshore pipelines.

With reference to FIG. 1, a hydrocarbon fluid may be cooled subsea bypassing the hydrocarbon fluid through the heat exchanger 16 which isdisposed in heat exchange relationship with the heat exchange circuit18. The hydrocarbon fluid transfers thermal energy to the cooling mediumfluid which is circulated through the heat exchange circuit 18, therebycooling the hydrocarbon fluid and heating the cooling medium fluid.

The heated cooling medium fluid is subsequently cooled by diverting theheated cooling medium fluid from the heat exchange circuit 18 into thesubsea cooling unit 22. The heated cooling medium fluid enters thesubsea cooling unit 22 through an inlet 24 and is passed into one ormore subsea cooling modules 28 via a first conduit 30. The subseacooling module(s) 28 are in heat exchange relationship with thesurrounding seawater and therefore thermal energy in the heated coolingmedium fluid is transferred to the surrounding seawater as coolingmedium fluid is passed through the cooling module(s) 28. The cooledcooling medium fluid is then redirected from the cooling module(s) 28 tothe heat exchange circuit 18 through outlet 26 via second conduit 32.

Another embodiment of the invention is described with reference to FIG.2, in which the cooling medium fluid from the subsea cooling module(s)28 is used as a heat transfer medium to alternately cool and then heattwo hydrocarbon process fluids. A hot hydrocarbon fluid from a wellheadis passed through heat exchanger 16 which is disposed in heat exchangecircuit 18. The hydrocarbon fluid transfers thermal energy to thecooling medium fluid which is circulated through the heat exchanger 16,thereby cooling (and possible condensing, at least in some part) thehydrocarbon fluid and heating the cooling medium fluid.

The cooled hydrocarbon fluid is then passed to separator 38 andseparated into a hydrocarbon gas component and a condensate (alsoincluding water). The hydrocarbon gas component is passed throughconduit 40 to export or further processing, with optional glycolinjection in conduit 40.

The condensate is passed through conduit 42 via pump 44 to heatexchanger 16′ which is also disposed in heat exchange circuit 18. Theheated cooling medium fluid transfers thermal energy to the condensate.The re-heated condensate is subsequently passed through conduit 46 toexport or further processing.

The partly cooled cooling medium fluid is subsequently further cooled bydiverting the heated cooling medium fluid from the heat exchange circuit18 into the subsea cooling unit 22, as shown in FIG. 1. The coolingmedium fluid enters the subsea cooling unit 22 through an inlet 24 andis passed into one or more subsea cooling modules 28 via a first conduit30. The subsea cooling module(s) 28 are in heat exchange relationshipwith the surrounding seawater and therefore thermal energy in the heatedcooling medium fluid is transferred to the surrounding seawater ascooling medium fluid is passed through the cooling module(s) 28. Thecooled cooling medium fluid is then redirected from the coolingmodule(s) 28 to the heat exchange circuit 18 through outlet 26 viasecond conduit 32.

In this particular embodiment, the subsea cooling system 10 of thepresent invention, may be employed to both cool a hydrocarbon stream andto heat a further process stream, such as the condensate streamdescribed above. It will be appreciated by persons skilled in the artthat the subsea cooling system 10 may be configured in alternativearrangements to utilise both cooling and heating capacity of the coolingmedium fluid, so that a second (or subsequent) hydrocarbon process fluidmay be heated or cooled. Appropriate insulation will be applied and pipelengths minimised on connecting pipes where it is desired to maintainthe heat in the heated cooling medium.

It will be readily apparent to a person skilled in the relevant art thatthe present invention has significant advantages over the prior artincluding, but not limited to, the following:

-   -   Absolute and complete temperature control of the hydrocarbon        fluid is possible, using the cooling medium pump to control the        flow of cooling medium fluid through the subsea cooling unit and        system. An operator can select a desired temperature at which        the operator wishes to apply subsea processing and any        separation steps.    -   There is no direct use of seawater. Seawater is a difficult        fluid to work with due to its corrosiveness, as well as issues        associated with entrained marine life, fouling, and scale. In        contrast to prior art hydrocarbon cooling systems, the present        invention avoids any pumping or handling of seawater in any        area. The subsea cooling system is a fully sealed, clean,        non-corrosive environment with no direct contact with external        seawater. The cooling medium fluid is circulated by a single        phase pump acting within the sealed subsea cooling system.    -   The cooling medium fluid is inert and may be the same as aqueous        subsea hydraulic control fluids; hence no additional fluid is        required for a subsea hydrocarbon processing facility comprising        the subsea hydrocarbon cooling system of the present invention.    -   There is no direct negative effect from small amounts of marine        growth or scale. Other devices which use seawater velocity to        attempt to control heat transfer and temperature are likely to        suffer degraded performance in this regard. While some reduction        in total heat transfer capacity will result from marine fouling        or scale, this can be accommodated by adding total heat exchange        area in the cooling module in the design stage; importantly the        ability to control temperature will not be affected.    -   Any number of required subsea cooling modules 26 may be        installed in the subsea region depending on the cooling        requirements of the subsea hydrocarbon processing area 12. The        cooling capacity of the system 10 of the present invention can        be scaled according to need.    -   The cooling modules 26 may be constructed of carbon steel,        ensuring cost savings. Standard cathodic protection, such as        zinc anodes, in addition to a minimal coating system, can be        provided on the external surfaces of the cooling modules 26 in a        similar fashion to how existing carbon steel export pipelines        and steel jackets are protected from corrosion in seawater.    -   The subsea hydrocarbon cooling system of the present invention        allows for temperature control of the hydrocarbon fluid. Cooling        may be accurately controlled from ‘fully off’ to maximum cooling        as per design requirements.    -   Cooling does not involve any valves or by-pass operation of the        hydrocarbon stream. Temperature control is achieved by        controlling the flow of the cooling medium fluid which        significantly simplifies the process.    -   There is a reduced risk of hydrate formation and blockages        associated with wax or sand build by employing the subsea        hydrocarbon cooling system of the present invention.    -   The subsea cooling module is likely to be fabricated from carbon        steel, and hence significantly lower capital costs are likely to        be incurred in comparison with those incurred with conventional        systems which directly cool and contain the hydrocarbon fluid in        a subsea environment.

Numerous variations and modifications will suggest themselves to personsskilled in the relevant art, in addition to those already described,without departing from the basic inventive concepts. All such variationsand modifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription.

It is to be understood that, although prior art use and publications maybe referred to herein, such reference does not constitute an admissionthat any of these form a part of the common general knowledge in theart, in Australia or any other country.

For the purposes of this specification it will be clearly understoodthat the word “comprising” means “including but not limited to”, andthat the word “comprises” has a corresponding meaning.

1. A subsea hydrocarbon cooling system, said system comprising: one ormore hydrocarbon process fluid heat exchangers arranged in heat exchangecommunication between a hydrocarbon process fluid and a single phasecooling medium fluid, wherein the one or more hydrocarbon process fluidheat exchangers are located subsea; a cooling medium fluid distributionpipe system connected to the hydrocarbon process fluid heat exchangers;and, a subsea cooling unit for cooling the cooling medium fluid, thesubsea cooling unit comprising: an inlet and an outlet arranged in fluidcommunication with the cooling medium fluid distribution pipe system; aflow controller to control the flow of cooling medium fluid in saidsystem; one or more subsea cooling modules, each cooling modulecomprising a plurality of cooling pipes configured in heat exchangerelationship with surrounding seawater; a first conduit for providingfluid communication between the inlet and the one or more subsea coolingmodules; and a second conduit for providing fluid communication betweenthe one or more subsea cooling modules and the outlet.
 2. The systemaccording to claim 1, wherein the one or more subsea cooling modules arelocated above the sea bed.
 3. The system according to claim 2, whereinthe one or more subsea cooling modules are supported by one or morerespective mud mats residing on the sea bed.
 4. The system according toclaim 3, wherein the one or more respective mud mats comprise a pallet.5. The system according to claim 4, wherein the one or more respectivemud mats incorporate lengths of pipe through which the cooling mediumfluid is circulated and/or distributed to the one or more coolingmodules.
 6. The system according to claim 1, wherein the first andsecond conduits are arranged in fluid communication with the coolingmodules via respective seabed pipes.
 7. The system according to claim 1,wherein the flow controller comprises a pump or a flow control valve. 8.A subsea cooling unit for use in a hydrocarbon process fluid coolingsystem, said unit being adapted to cool a single phase cooling mediumfluid circulating through a subsea heat exchange circuit, the subseaheat exchange circuit having a cooling medium fluid distribution pipesystem, the subsea cooling unit comprising: an inlet and an outletarranged in fluid communication with the cooling medium fluiddistribution pipe system; a flow controller to control the flow ofcooling medium fluid in the subsea heat exchange circuit; one or moresubsea cooling modules, each cooling module comprising a plurality ofcooling pipes configured in heat exchange relationship with surroundingseawater; a first conduit arranged to provide fluid communicationbetween the inlet and the one or more subsea cooling modules; and asecond conduit arranged to provide fluid communication between the oneor more subsea cooling modules and the outlet.
 9. The subsea coolingunit according to claim 8, wherein the plurality of cooling pipes areconfigured in a manifold of parallel lengths of pipe.
 10. The subseacooling unit according to claim 8, wherein the plurality of coolingpipes are configured in a coiled arrangement.
 11. The subsea coolingunit according to claim 8, wherein the one or more subsea coolingmodules is coiled around at least part of a substructure of an offshoreplatform.
 12. The subsea cooling unit according to claim 8, wherein theone or more subsea cooling modules is coiled around a subsea facility.13. The subsea cooling unit according to claim 8, wherein the one ormore subsea cooling modules is provided with an open-ended shroud todirect flow of seawater over the subsea cooling modules and/or increasethermal convection from the subsea cooling modules.
 14. The subseacooling unit according to claim 13, wherein the shroud is provided withbaffles, vanes or means for increasing and/or directing flow of seawaterover the subsea cooling modules.
 15. A method of cooling a hydrocarbonprocess fluid, said method comprising: passing the hydrocarbon processfluid through a subsea hydrocarbon process fluid heat exchanger andpassing a single phase cooling medium fluid through a cooling mediumfluid distribution pipe system, wherein said subsea heat exchanger andsaid pipe system are configured to provide heat exchange communicationbetween the hydrocarbon process fluid and the cooling medium fluid,thereby producing a cooled hydrocarbon fluid and a heated cooling mediumfluid; and, cooling the heated cooling medium fluid by passing theheated cooling medium fluid through a subsea cooling unit, wherein thesubsea cooling unit comprises: an inlet and an outlet arranged in fluidcommunication with the cooling medium fluid distribution pipe system; aflow controller to control the flow of cooling medium fluid through asubsea heat exchange circuit; one or more subsea cooling modules, eachcooling module comprising a plurality of cooling pipes configured inheat exchange relationship with surrounding seawater; a first conduitarranged to provide fluid communication between the inlet and the one ormore subsea cooling modules; and a second conduit arranged to providefluid communication between the one or more subsea cooling modules andthe outlet.
 16. The method according to claim 15, wherein prior topassing the heated cooling medium fluid through the subsea cooling unit,said method comprises passing the heated cooling medium fluid throughsaid pipe distribution system and passing a second hydrocarbon processfluid through a second subsea heat exchanger, wherein said pipedistribution system is configured in a heat exchange relationship withthe second subsea heat exchanger.
 17. The method according to claim 16,wherein the cooling medium fluid is cooled by thermal exchange with thesecond hydrocarbon process fluid.
 18. The method according to claim 16,wherein the cooling medium fluid is heated by thermal exchange with thesecond hydrocarbon process fluid.